The copper-tungsten alloy is a material endowed with high electric conductivity of copper and arc erosion resistance of tungsten, which has been used from olden times as electrode material and electric contact. Recently, it has come to be employed as a package material for semiconductors and heat sink members in the electronic field.
Because copper and tungsten do not undergo solid solution, the copper-tungsten alloy is manufactured by powder metallurgy process. The manufacturing method by this powder metallurgy process is roughly divided into two: infiltration process and powder mixing process. The infiltration process consists in press-molding and sintering only the tungsten powder to make a porous tungsten skeleton in advance, into which molten copper is impregnated. The powder mixing process press-molds and sinters the powder mixture of copper powder and tungsten powder in a prescribed proportion.
Both the infiltration process and powder mixing process are however problematical. What are at issue for the infiltration process are the difficulty in adjusting the porosity of the tungsten skeleton, that the content of copper is likely to disperse and that this process requiring many processes ends up being costly. In the powder mixing process, the tungsten powder and copper powder are in a compaction state when they are compounded. Since reducing gas does not infiltrate satisfactorily into the green compact when sintering, the surface of the tungsten particle cannot be reduced sufficiently. The wettability of the copper powder in a liquid phase thus worsens against the tungsten powder causing thereby the voids.
Since the copper powder itself is not reduced enough, the remaining oxygen causes its thermal conductivity to diminish.
The powder mixing method is further problematical in that, when copper powder and tungsten powder are mixed, highly ductile copper is likely to aggregate causing thus a copper pool or pore in the sintered body. In general, fine alloy has thus far hardly been obtained from these materials.
Although the copper aggregate produced in the mixing of powder may be powdered down by long-hour milling (for example, 120-hour milling with ball mill), this long-time milling admits impurities to mingle into the compound from the inner wall of the recipient to reduce the electrical and thermal conductivity, which is another problematical point.
To get a good powder mixture of copper and tungsten, an attempt was made to compound, for instance, tungsten oxide with copper oxide and reduce this mixture. Because this process is accompanied by higher reduction temperature of tungsten, the copper results in re-aggregation. Further, the mixed powder thus obtained becomes less soft, which makes it difficult to treat it in the subsequent processes.
It is therefore a general object of this invention to provide a copper-tungsten alloy which is homogeneous and highly dense, excellent in thermal conductivity and transverse rupture strength improving the wettability of copper in the liquid phase to the surface of tungsten particle by adding phosphor and removing the oxygen in copper powder as well as the manufacturing process for this alloy.
It is still a more specific object of this invention to manufacture a powder mixture where the copper is uniformly dispersed without being aggregated and the impurities due to milling are little to get high quality copper-tungsten alloy from this powder mixture.